Simplify calculation of week number

This also removes a dependency to 3rd party licensed code.

Backport of a29b7635bd1d58b29fca96bd3e7831d0ee1f6666 in Qt 5.

Change-Id: I647457d7787eed6d5bfc31de4816e68a9f236239
Reviewed-by: Simon Hausmann <[email protected]>

Author: laknoll

src/corelib/tools/qdatetime.cpp

@@ -453,22 +453,6 @@ int QDate::daysInYear() const
     January 2000 has week number 52 in the year 1999, and 31 December
     2002 has week number 1 in the year 2003.
 
-    \legalese
-    Copyright (c) 1989 The Regents of the University of California.
-    All rights reserved.
-
-    Redistribution and use in source and binary forms are permitted
-    provided that the above copyright notice and this paragraph are
-    duplicated in all such forms and that any documentation,
-    advertising materials, and other materials related to such
-    distribution and use acknowledge that the software was developed
-    by the University of California, Berkeley.  The name of the
-    University may not be used to endorse or promote products derived
-    from this software without specific prior written permission.
-    THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR
-    IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
-    WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
-
     \sa isValid()
 */
 
@@ -478,46 +462,29 @@ int QDate::weekNumber(int *yearNumber) const
         return 0;
 
     int year = QDate::year();
-    int yday = dayOfYear() - 1;
+    int yday = dayOfYear();
     int wday = dayOfWeek();
-    if (wday == 7)
-        wday = 0;
-    int w;
-
-    for (;;) {
-        int len;
-        int bot;
-        int top;
-
-        len = isLeapYear(year) ? 366 : 365;
-        /*
-        ** What yday (-3 ... 3) does
-        ** the ISO year begin on?
-        */
-        bot = ((yday + 11 - wday) % 7) - 3;
-        /*
-        ** What yday does the NEXT
-        ** ISO year begin on?
-        */
-        top = bot - (len % 7);
-        if (top < -3)
-            top += 7;
-        top += len;
-        if (yday >= top) {
+
+    int week = (yday - wday + 10) / 7;
+
+    if (week == 0) {
+        // last week of previous year
+        --year;
+        week = (yday + 365 + (QDate::isLeapYear(year) ? 1 : 0) - wday + 10) / 7;
+        Q_ASSERT(week == 52 || week == 53);
+    } else if (week == 53) {
+        // maybe first week of next year
+        int w = (yday - 365 - (QDate::isLeapYear(year + 1) ? 1 : 0) - wday + 10) / 7;
+        if (w > 0) {
             ++year;
-            w = 1;
-            break;
+            week = w;
         }
-        if (yday >= bot) {
-            w = 1 + ((yday - bot) / 7);
-            break;
-        }
-        --year;
-        yday += isLeapYear(year) ? 366 : 365;
+        Q_ASSERT(week == 53 || week == 1);
     }
+
     if (yearNumber != 0)
         *yearNumber = year;
-    return w;
+    return week;
 }
 
 #ifndef QT_NO_TEXTDATE